Coating processes – Interior of hollow article coating – Coating by vapor – gas – mist – or smoke
Reexamination Certificate
1998-08-21
2001-01-30
Beck, Shrive (Department: 1762)
Coating processes
Interior of hollow article coating
Coating by vapor, gas, mist, or smoke
C427S239000, C427S255310, C427S255390, C427S248100, C427S253000, C118S728000, C118SDIG001, C118SDIG001
Reexamination Certificate
active
06180170
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an apparatus and a method for preparing and/or coating the surfaces of metallic hollow workpieces or structural elements, which comprise at least two connection openings between their outer and inner surfaces.
BACKGROUND INFORMATION
EP 0,349,420 discloses a method with an apparatus for the preparation and/or coating of the surfaces of metallic hollow structural elements, which comprise at least two connection openings between their outer and inner surfaces, especially for hollow blades in the field of turbine engine construction. In the disclosed method and apparatus, a cleaning gas mixture or a coating gas mixture is generated beneath a blade in a reaction space. The blade hangs in the reaction space, from which the outer surfaces may be cleaned or coated. The reaction gas first flows over the outer surfaces in one direction and then flows through a first opening in the hollow blade into the hollow spaces and past the inner surfaces, and finally flows out of the hollow spaces through a second opening in the hollow blade into an exhaust conduit for removal or return flow of the residual gases of the reaction gas.
Such apparatus and methods have the disadvantage that the concentration of individual reaction components, which are contained in the reaction gas and which react with the surfaces, diminishes along the path over the outer surfaces, the first opening, the inner surfaces, and up to the outlet out of the second opening, to such an extent that substantial reaction differences arise between the outer and inner surfaces and over the course of the inner surfaces.
The reaction differences between the outer and inner surfaces are partially overcome by measures as are described in the patents DE 4,035,789 and DE 4,119,967. However, it can be determined that the differences over the course of the inner surfaces from the entry into the hollow spaces up to the point of flowing out of the hollow spaces cannot be substantially improved using the prior methods. Moreover, the improved methods and apparatus have the disadvantage that they require retort structures that are constructed in an extremely complex and only slightly variable manner, and appear to be unsuitable for use in mass production.
A further essential disadvantage of the prior art is that the known apparatus and methods do not permit the use of different gas sources for the treatment of the outer and inner surfaces.
SUMMARY OF THE INVENTION
Objects of the invention are to overcome the disadvantages of the prior art. These objects are achieved according to the invention, insofar as a method is involved, by the following method steps:
a) preparing at least first and second reaction gas mixtures (I, II) by means of reaction gas sources for treating the outer and inner surfaces of the hollow structural elements,
b) passing the first reaction gas mixture (I) over the outer surfaces and then over the inner surfaces of the structural elements,
c) passing the second reaction gas mixture (II) over the inner surfaces and then over the outer surfaces of the structural elements.
In comparison to the prior methods, this present inventive method has the advantage, of providing the same uniform reaction effect of the reaction gases on and completely along the inner surfaces of the same hollow structural elements, whereby it achieves a greater uniformalization of the reaction results both for a preparation such as the reduction of sulfide-based or oxide-based surface contaminants as well as for a coating of the inner surfaces with protective layers that provide protection against oxidation, corrosion or sulfidation. In the event that the inner surfaces form channels, as are known in hollow turbine or compressor blades, then twice the channel length can be cleaned or coated in comparison to the cleaning or coating using typical methods, since the reaction gases can flow through the hollow spaces not only in one direction, but rather from two mutually opposed directions in sequence after one another.
In a preferred manner of carrying out the method, the first and second reaction gas mixtures (I, II) are composed of similar components, and the flow direction of the reaction gases is repeatedly reversed multiple times over the surfaces of the hollow structural element by respectively discontinuing and sequentially repeating the steps b) and c). This interval method especially has the advantage, in connection with inner surfaces that comprise protrusions and other obstacles, that reduced reaction effects, for example between the windward and leeward sides of the obstacle, can be counteracted. Another advantage is that higher flow velocities can be used since the windward and leeward side effects will compensate each other. In other words, the previously typical slow creeping velocities used for the throughflow of inner surfaces to avoid the formation of differences between the windward and leeward sides of obstacles, which can lead to a premature depletion of the reaction components, no longer need to be maintained, so that firstly the premature depletion of reaction components is overcome, and secondly a high uniformity of the preparation and/or the coating is achieved, which is especially provable in the case of coatings by measuring the coating thickness. Finally, the duration of the method is reduced with this variation of the method, if the same preparation and/or coating results are to be achieved as with typical methods or apparatus.
In a further preferred manner of carrying out the method, at least one of the reaction gas sources provides reaction gases, and preferably halogen-containing gases, that serve for cleaning the outer and inner surfaces. Among these halogen-containing gases, especially chlorine-containing or fluorine-containing gases have proved themselves suitable, which gases have an etching reaction effect on the surfaces to be cleaned.
The reaction gas sources do not always need to be of the same type. In the case of surface preparations, at least one of the reaction gas sources preferably supplies reaction gases that serve to reduce sulfide-based or oxide-based deposits on the outer and inner surfaces, whereby such reaction gases are preferably hydrogen-containing gases, which flow around the surfaces of the structural elements in a preferred direction, while a coating source of a different type is effective for providing a coating gas to flow in the opposite direction. Flushing gases for cleaning an apparatus, before treated structural elements are removed from the apparatus can also flow in a preferred direction around the surfaces in the reaction spaces, for example in order to drive poisonous components in the preferred direction. Furthermore, connection holes between outer and inner surfaces of the structural elements, as they are known as film cooling holes in turbine blades, can be kept clear of undesired deposits and undesired contaminants during a cool-down phase after a coating process, by means of an inert gas flowing through the structural elements in the direction of the second reaction gas mixture II, from inside to outside through the connection holes during the cool-down phase.
Consequently, the second reaction gas (II) can be a coating reaction gas, such as preferably a chromizing or aluminizing reaction gas, a reducing gas such as preferably a hydrogen-containing gas, or an inert gas. In this context, the inert gas is preferably used during the phase of heating-up or of cooling-down.
During a gas diffusion coating of the outer or inner surfaces, preferably halide-containing gases will be decomposed on the metallic outer or inner surfaces of the hollow structural elements, into a metallic component that is deposited as a coating onto the outer and inner surfaces, and a halogen component that can be reused as an activator. The depletion of the metal source and the thinning of the reaction gas is especially grave at the flow velocities of typical methods, and has a negative effect on the uniformalization of the layer thicknesses, which is overcome by the
Grossmann Valentin
Pillhoefer Horst
Thoma Martin
Beck Shrive
Chen Bret
Fasse W. F.
Fasse W. G.
MTU Motoren - und Turbinen-Union Muenchen GmbH
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